1. Field of the Invention
The present invention relates to a dual-rated current transformer circuit, and more particularly to a miniature dual-rated transformer circuit for use in a protective relay.
2. Discussion of the Prior Art
The use of protective relays for or in a circuit breaker or other electrical apparatus is well known. Traditionally, the relay detects a condition and generates a signal to operate, for example, a trip coil in a low ampere industrial rated circuit breaker. A current transformer assembly is utilized to provide operating power to the trip coil. Traditionally, a single current transformer core fits within the circuit breaker and supplies a sufficient current and operating power to the trip unit circuit for a number of ampere ratings.
One manner of decreasing size constraints but still allowing a circuit to operate over a wide range of ampere ratings is to utilize a fixed transformer size and a fixed secondary winding thereon. The number of primary turns are varied inversely with the circuit ampere rating. See U.S. Pat. No. 5,015,983, assigned to the assignee of the present invention. However, varying the number of primary turns in a current transformer circuit will not allow different input current ratings to produce the same current through the primary winding.
Moreover, larger breakers for industrial or utility applications traditionally utilize protective relays that have their own enclosures. The protective relays have a source of power to operate other than from the current transformer. The output of the protective relay is normally a contact or solid-state device to connect the trip coil to a source of power independent from the relay. For this application, the current transformers are used to replicate and isolate the input current and are normally rated one ampere or five amperes. The current transformer must work over a large current range that includes fault current, which is much greater than rated current for protection and metering, and metering current which can be less than rated current. Traditionally, a typical current transformer for a one ampere input rating would have a twenty turn primary and a separate design for a five ampere input rating which would have four turns.